fix #470, string interpolation now respects Dart toString

test/codegen/expect/BenchmarkBase.js

 dart_library.library('BenchmarkBase', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const BenchmarkBase$ = Object.create(null);
   BenchmarkBase$.Expect = class Expect extends core.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
-        dart.throw(`Values not equal: ${expected} vs ${actual}`);
+        dart.throw(dart.str`Values not equal: ${expected} vs ${actual}`);
       }
     }
     static listEquals(expected, actual) {
       if (expected[dartx.length] != actual[dartx.length]) {
-        dart.throw(`Lists have different lengths: ${expected[dartx.length]} vs ${actual[dartx.length]}`);
+        dart.throw(dart.str`Lists have different lengths: ${expected[dartx.length]} vs ${actual[dartx.length]}`);
       }
       for (let i = 0; i < dart.notNull(actual[dartx.length]); i++) {
         BenchmarkBase$.Expect.equals(expected[dartx.get](i), actual[dartx.get](i));
       }
     }
     fail(message) {
       dart.throw(message);
     }
   };
   dart.setSignature(BenchmarkBase$.Expect, {
@@ skipping 38 matching lines @@
         this.warmup();
       }), 100);
       let result = BenchmarkBase$.BenchmarkBase.measureFor(dart.fn(() => {
         this.exercise();
       }), 2000);
       this.teardown();
       return result;
     }
     report() {
       let score = this.measure();
-      core.print(`${this.name}(RunTime): ${score} us.`);
+      core.print(dart.str`${this.name}(RunTime): ${score} us.`);
     }
   };
   dart.setSignature(BenchmarkBase$.BenchmarkBase, {
     constructors: () => ({new: [BenchmarkBase$.BenchmarkBase, [core.String]]}),
     methods: () => ({
       run: [dart.void, []],
       warmup: [dart.void, []],
       exercise: [dart.void, []],
       setup: [dart.void, []],
       teardown: [dart.void, []],
       measure: [core.double, []],
       report: [dart.void, []]
     }),
     statics: () => ({measureFor: [core.double, [core.Function, core.int]]}),
     names: ['measureFor']
   });
   // Exports:
   exports.BenchmarkBase = BenchmarkBase$;
 });